In recent years, energy consumption has dramatically increased with growth of industries. In the future, further increase in demand for energy is expected. Against this backdrop, today's demands are placed on the development of production technology of economic and high-performance clean energy which does not put loads the Earth's environment. Among all, a solar cell which utilizes unlimited sunlight has attracted attention as a new energy source. Most of commercialized solar cells are inorganic solar cells using single crystal silicon, polycrystal silicon or amorphous silicon. However, since these inorganic silicon-based solar cells are produced in complicated processes at high costs, the solar cells have not widespread among ordinary households. To solve such drawbacks, an organic solar cell using organic materials which achieves lower costs and large dimensions in simple processes has actively researched.
In research of such organic solar cells, Professor Gratzel of Universite de Lausanne in Switzerland discloses a dye-sensitized solar cell as a kind of organic solar cell based on photochemical reaction using porous titanium oxide, ruthenium pigment, iodine and iodine ion, which has high conversion efficiency of 10% (B. O'Regan, M. Gratzel, Nature, 353, 737 (1991)).
Concerning an organic thin film solar cell as another kind of organic solar cell, it is reported that a low-molecular organic thin film solar cell which is formed using an electron-donating material (donor material) and an electron-accepting material (acceptor material) as low-molecular materials according to a vacuum evaporation method achieves conversion efficiency of 3.6% (P. Peumans and S. R. Forrest, Appl. Phys. Lett. 79, 126 (2001)).
Use of a polymeric material for a photoactive layer which receives light and generates electric power has been considered. In this case, since the costly vacuum evaporation method is not used for forming the photoactive layer, further reduction in costs can be expected.
It is reported that a mixed film of a conjugated polymer and a fullerene derivative achieves conversion efficiency of 2.5% (S. E. Shaheen, Appl. Phys. Lett. 78, 841 (2001)). With this as a turning point, research of the organic solar cell attracts attention again and various ideas are devised to obtain an organic solar cell having high efficiency.
For example, the following ideas are devised. Irregularity is made on a backside cathode for collecting electrons to improve confinement of light and collection of electrons (M. Niggemann, e-MRS 2003, oral presentation (2003)). Alternatively, by preparing an organic solar cell using a material having high hall mobility as a conjugated polymer and then properly humidifying the organic solar cell, rearrangement of the conjugated polymer and proper mixed state of a hall transport material and an electron transport material are achieved, thereby improving charge separation (F. Padinger, Adv. Funct. Mater. 13, 85 (2003)). Conversion efficiency of 3.5% is obtained according to the above-mentioned techniques.
In addition, to improve efficiency of the organic thin film solar cell, stack of photoactive layers has been actively addressed. In the organic thin film solar cell using a low-molecular material, by separating functions by layer, providing a Wiston-type light collection structure on a glass substrate on the incident light side (P. Peumans, V. Bulovic and S. R. Forrest, Appl. Phys. Lett. 76, 2650 (2000)) or stacking photoactive parts with a metal layer of about 0.5 to 5 nm being interposed (A. Yakimov and I. R. Forrest, Appl. Phys. Lett. 80, 1667 (2002)), open end voltage (Voc) is substantially doubled. Also in the case of using a polymeric material, a plurality of photoactive layers are stacked. As described above, stack of photoactive layer is demonstrated to be one of the most effective techniques for improving efficiency of the organic thin film solar cell.
However, in the case where the photoactive layer is formed by being coated with a solution of an organic compound containing a donor material and an acceptor material, when a second photoactive layer is stacked to a first photoactive layer, the first photoactive layer may melt due to a solvent for forming the second photoactive layer, thereby causing destruction of structure of the first photoactive layer or deteriorating in functions. For this reason, it is disadvantageously difficult to form stacked structure of the photoactive layers.
Japanese Examined Patent Publication No. 8-31616 discloses a multilayer organic solar cell in which a metal layer is interposed between a plurality of photoactive layers formed according to a deposition method or the like. When the photoactive layers are formed by solution coating in this manner, by interposing the metal layer between the photoactive layers, the metal layer can prevent the solvent used for forming the second photoactive layer from penetrating into the first photoactive layer. Consequently, it is prevented from destruction of structure of the first photoactive layer or deterioration in functions due to the solvent. However, in the case where the metal layer is provided between the photoactive layers, when a thickness of the metal layer is thick, light transmittance is lowered and thus, power conversion efficiency of the solar cell is lowered. For this reason, the metal layer needs to have a very small thickness. However, when the thickness of the metal layer is decreased, the solvent is easy to penetrate into and act upon the first photoactive layer. Japanese Unexamined Patent Publication No. 2001-319698 discloses a multilayer organic solar cell in which a single cell formed of a conductive layer, an under coating layer, a photosensitive layer (photoactive layer), a charge transfer layer, a counter electrode conductive layer which are deposited in this order are stacked via a support body made of glass or the like. With such structure, since each cell can be independently formed, deterioration in forming the photoactive layer by solution coating does not occur. However, many layers and complicated structure may cause an increase in production costs and a decrease in light transmittance, leading to deterioration of power conversion efficiency of the solar cell.